1. Field of the Invention
The present invention is related to a method of treatment with a microwave plasma. More specifically, the invention is related to a method of treatment with a microwave plasma, which is capable of forming a plasma in a short period of time maintaining stability in conducting the treatment with a microwave plasma in order to chemically deposit a film on a substrate.
2. Description of the Related Art
The chemical vapor deposition (CVD) is a technology for precipitating a reaction product like a film on the surface of a substrate relying upon the vapor growth in a high-temperature atmosphere by using a starting material gas that does not undergo a reaction at normal temperature. The CVD has been widely employed for the manufacture of semiconductors and for reforming the surfaces of metals and ceramics. In recent years, the CVD has been employed even for reforming the surfaces of plastic containers and, particularly, for improving gas barrier properties.
A plastic container having a deposited carbon film has been know already as a plastic material with a chemically deposited film.
Japanese Unexamined Patent Publication (Kokai) No. 53116/1996 discloses a plastic container coated with a carbon film characterized in that a hard carbon film is formed on the inner wall surface of a container formed of a plastic material.
The plasma CVD is a process for growing a thin film by utilizing a plasma. That is, the plasma CVD basically comprises decomposing a gas containing a staring material gas with electric energy of an intense electric field under a reduced pressure, to deposit, on a substrate, a substance that is formed through a chemical reaction in the gaseous phase or on the substrate.
The plastic state is realized by a glow discharge. Depending upon the systems of the glow discharge, there have been known a method that utilizes a DC glow discharge, a method that utilizes a high-frequency glow discharge and a method that utilizes a microwave discharge.
In the coated plastic container based upon the above-mentioned prior art, a carbon film is deposited on the inner wall surface of the container based on the high-frequency glow discharge by using a so-called capacity-coupled CVD apparatus having an internal electrode arranged in the container and an external electrode arranged outside the container accompanied, however, by such problems that the constitution of the apparatus is complex and the operation becomes complex, too.
With the microwave plasma CVD which utilizes a microwave discharge in a chamber, on the other hand, neither the external electrode nor the internal electrode is necessary, and the apparatus can be constituted very simply. Besides, the pressure in the apparatus needs be reduced to such a degree that the microwave electric discharge generates in the plastic container only, and there is no need of maintaining a high degree of vacuum in the whole apparatus offering advantages such as easy operation and excellent productivity.
According to the study conducted by the present inventors, however, the treatment with a microwave plasma involves a considerable degree of time lag (for example, about 10 seconds) between the introduction of microwaves and the generation of plasma, the time lag being not constant but subject to change to a considerable degree depending upon each treatment. Therefore, difficulty is involved in controlling the treatment conditions and the effect of treatment is not stable, either.
It is therefore an object of the present invention to provide a method of treatment with a microwave plasma, which is capable of forming a plasma in a short period of time maintaining stability in conducting the treatment with a microwave plasma, i.e., capable of generating a plasma within a very short period of time from when the microwaves are introduced into a chamber for treatment with a plasma, the state of the generated plasma being stable, and of stably conducting the treatment.
Another object of the present invention is to provide a method of treatment with a microwave plasma capable of forming a film having particularly excellent gas shut-off property (gas barrier property) on, for example, the inner surface and/or the outer surface of the container.
According to the present invention, there is provided a method of treatment with a microwave plasma by maintaining a reduced pressure in a plasma-treating chamber for treatment with a plasma in which a substrate that is to be treated is contained, introducing a treating gas into the plasma-treating chamber and introducing microwaves into the plasma-treating chamber, wherein a metallic antenna is disposed in the plasma-treating chamber to generate a plasma in a short period of time maintaining stability.
According to the method of the present invention, it is desired that:
1. The metallic antenna has a length which is not smaller than 0.02 times as long as the wavelength of the microwaves;
2. A supply pipe for supplying a treating gas is extending in the plasma-treating chamber, the metallic antenna is so disposed as to be directed outward from an end of the supply pipe;
3. Use is made of a metallic pipe for supplying a treating gas, the pipe being also used as a metallic antenna;
4. The substrate to be treated is a plastic substrate;
5. The treating gas is a carbon-source gas or a gas containing an organosilicon compound and oxygen;
6. The substrate to be treated is a plastic container, the interior and/or the exterior of the plastic container is maintained in a reduced-pressure atmosphere containing a treating gas, and a microwave discharge is produced inside of the container and/or outside of the container in order to chemically deposit a film on the inner surface of the container and/or on the outer surface of the container;
7. The plastic container is held in the plasma-treating chamber, the exterior of the plastic container and the interior of the plastic container are maintained in an air-tight state, the interior of the plastic container is maintained in a reduced-pressure condition in which a microwave discharge takes place in a state where a treating gas is introduced into the plastic container, the exterior of the plastic container is maintained in a reduced-pressure condition in which no microwave discharge takes place in a state where the treating gas is introduced into the plastic container, and microwaves are introduced into the exterior of the plastic container in the plasma-treating chamber; and
8. A microwave reflector is disposed in the plasma-treating chamber so as to be opposed to the bottom of the plastic container.
In the present invention, further, it is desired that an end of the metallic antenna is positioned on an extension in the horizontal direction from an upper end or a lower end of a microwave introduction port formed in the plasma-treating chamber, or in the vicinity thereof.
It is further desired that on the surface of the metallic antenna is formed a film of the same kind as the film formed on the surface of the substrate by the treatment with a plasma. This effectively avoids a drop in the performance of the film on the surface of the substrate caused by sputtering that occurs during the treatment with a plasma.
In the present invention, further, it is desired that the supply pipe for supplying a treating gas is a porous pipe such as the one formed of a porous material having, for example, a nominal filtering precision of from 1 to 300 xcexcm and a pressure loss on the secondary side under the atmospheric pressure of from 0.01 to 25 KPa. By using such a porous pipe, it is allowed to form a film having excellent gas barrier property on the surface of the substrate. The nominal filtering precision stands for one of the characteristic values employed when a porous material is used as a filter. For example, a nominal filtering precision of 130 xcexcm stands for that when the porous material is used as the filter, foreign matters having particle diameters of not smaller than 130 xcexcm can be trapped.